# std::valarray<T>::operator[]

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< cpp‎ | numeric‎ | valarray

const T& operator[]( std::size_t pos ) const; | (1) |
T& operator[]( std::size_t pos ); | (2) |
std::valarray<T> operator[]( std::slice slicearr ) const; | (3) |
std::slice_array<T> operator[]( std::slice slicearr ); | (4) |
std::valarray<T> operator[]( const std::gslice& gslicearr ) const; | (5) |
std::gslice_array<T> operator[]( const std::gslice& gslicearr ); | (6) |
std::valarray<T> operator[]( const std::valarray<bool>& boolarr ) const; | (7) |
std::mask_array<T> operator[]( const std::valarray<bool>& boolarr ); | (8) |
std::valarray<T> operator[]( const std::valarray<std::size_t>& indarr ) const; | (9) |
std::indirect_array<T> operator[]( const std::valarray<std::size_t>& indarr ); | (10) |

Retrieve single elements or portions of the array.

The const overloads that return element sequences create a new std::valarray object. The non-const overloads return classes holding references to the array elements.

The selected elements(s) must exist:

- for overloads (1,2), if pos is not less than `size()`, the behavior is undefined; and
- for overloads (3-10), if the argument does not specify a valid subset of *this, the behavior is undefined.

### Parameters

pos | - | position of the element to return
slicearr | - | slice of the elements to return
gslicearr | - | gslice of the elements to return
boolarr | - | mask of the elements to return
indarr | - | indices of the elements to return

### Return value

1,2) A reference to the corresponding element.
3,5,7,9) A std::valarray object containing copies of the selected items.
4,6,8,10) The corresponding data structure containing references to the selected items.

### Exceptions

May throw implementation-defined exceptions.

### Notes

For proper std::valarray values a, b and proper std::size_t values i, j, all of the following expressions always evaluate to true:

1) (a[i] = q, a[i]) == q for non-const a
2) &a[i + j] == &a[i] + j

- This means that std::valarray elements are adjacent in memory.

3) &a[i] != &b[j] for every objects a and b that are not aliases of one another

- This means that there are no aliases in the elements and this property can be used to perform some kinds of optimization.

References become invalid on `resize()` or when the array is destructed.

For overloads (3,5,7,9), The function can be implemented with the return type different from std::valarray. In this case, the replacement type has the following properties:

  - All const member functions of std::valarray are provided.
  - std::valarray, std::slice_array, std::gslice_array, std::mask_array and std::indirect_array can be constructed from the replacement type.
  - For every function taking a const std::valarray<T>& except `begin()` and `end()`(since C++11), identical functions taking the replacement types shall be added;
  - For every function taking two const std::valarray<T>& arguments, identical functions taking every combination of const std::valarray<T>& and replacement types shall be added.
  - The return type does not add more than two levels of template nesting over the most deeply-nested argument type.

Slice/mask/indirect index accesses do not chain: v[v == n][std::slice(0, 5, 2)] = x; is an error because std::mask_array (the type of v[v == n]) does not have operator[].

### Example

Run this code

    #include <cstddef>
    #include <iomanip>
    #include <iostream>
    #include <valarray>

    int main()
    {
        std::valarray<int> data = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

        std::cout << "Initial valarray:   ";
        for (int n : data)
            std::cout << std::setw(3) << n;
        std::cout << '\n';

        data[data > 5] = -1; // valarray<bool> overload of operator[]
        // the type of data > 5 is std::valarray<bool>
        // the type of data[data > 5] is std::mask_array<int>

        std::cout << "After v[v > 5] = -1:";
        for (std::size_t n = 0; n < data.size(); ++n)
            std::cout << std::setw(3) << data[n]; // regular operator[]
        std::cout << '\n';
    }

Output:

    Initial valarray:     0  1  2  3  4  5  6  7  8  9
    After v[v > 5] = -1:  0  1  2  3  4  5 -1 -1 -1 -1

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 389 | C++98 | the return type of overload (1) was `T` | corrected to const T&
LWG 430 | C++98 | the behavior was unclear for overloads (3-10) if an invalid subset is specified | the behavior is undefined in this case

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